Particularly in recent years, tire manufacturers have been making considerable efforts to develop novel solutions to a problem dating from the very first use of tired wheels of the inflatable type, namely the problem of how to allow the vehicle to continue to travel despite a considerable or complete loss of pressure of one or more tires. For decades, the spare tire was considered to be the only and universal solution. Then, more recently, the substantial advantages associated with its possible omission have become apparent. The concept of “extended mobility” was developed. The associated techniques allow the same tire to run, depending on certain limits to be respected, after a puncture or a drop in pressure. This makes it possible for example to drive to a breakdown point without having to stop, often in hazardous circumstances, to fit the spare tire.
Self-sealing compositions that allow such an objective to be achieved, and which by definition are capable of automatically ensuring, that is to say without external intervention, that a tire is sealed in the event of a perforation of the latter by a foreign body, such as a nail, are particularly difficult to develop.
To be useable, a self-sealing layer must satisfy many conditions of a physical and chemical nature. In particular, it must be effective over a very wide range of operating temperatures, and be so throughout the lifetime of the tires. It must be capable of closing off the hole when the perforating object remains in place and, when the latter is expelled, said self-sealing layer must be able to fill the hole and seal the tire.
Many solutions have been devised but have not been able truly to be developed for vehicle tires, in particular through the lack of stability over time or lack of effectiveness under extreme operating temperature conditions, or else because of difficulties in manufacturing and/or using these self-sealing compositions.
Thus, to help to maintain good effectiveness at high temperature, document U.S. Pat. No. 4,113,799 (or FR-A-2 318 042) has proposed, as self-sealing layer, a composition comprising a combination of butyl rubbers of high and low molecular weights, which are partially crosslinked, optionally together with a small amount of a thermoplastic stirene elastomer.
Document U.S. Pat. No. 4,228,839 has proposed, as self-sealing layer for a tire, a rubber compound containing an irradiation-degradable first polymer material, such as polyisobutylene, and an irradiation-crosslinkable second polymer material, preferably a butyl rubber.
Document U.S. Pat. No. 4,426,468 has also proposed a self-sealing composition for a tire, based on a crosslinked butyl rubber of very high molecular weight.
A known drawback of butyl rubbers is that they suffer from large hysteresis losses (high tan δ level) over a wide temperature range, which drawback has repercussions for the self-sealing compositions themselves, with a large increase in hysteresis and an appreciable reduction in rolling resistance of the tires.
Self-sealing compositions based on unsaturated diene elastomers (natural rubber) have also been described, for the same type of application, in particular in the U.S. Pat. No. 4,913,209, U.S. Pat. No. 5,085,942 and U.S. Pat. No. 5,295,525.
These compositions are characterized by the combined presence of a high content of hydrocarbon resin as tackifier, always greater than 100 phr, and a large amount of elastomer (isoprene) in the liquid state.
Now, such a high resin content, apart from the fact that its incorporation requires very long kneading of the elastomeric matrix, may also be prejudicial to hysteresis and consequently to the rolling resistance of tires.
Furthermore, a large amount of liquid elastomer gives the composition a high fluidity, which is a source of other drawbacks, especially a risk of the self-sealing composition creeping when it is used at a relatively high temperature (typically above 60° C.), frequently encountered when certain tires are used.